1. Field of the Invention
The present invention relates to a radio communications system and a radio communications method for controlling transmission power of a shared control channel for transmitting control signals to a plurality of mobile stations.
2. Description of the Related Art
Recently, in the IMT-2000 (International Mobile Telecommunications-2000), studies have been conducted on a HSDPA (High Speed Downlink Packet Access) system as the radio communications control system described above which is a higher speed downlink packet transmission system having the purposes of achieving a higher peak transmission speed, a lower transmission delay and a higher throughput etc.
The HSDPA system is a transmission system for carrying out communications by sharing one physical channel among a plurality of mobile stations in a time-division manner. The HSDPA system assigns a channel to a mobile station of better radio quality at each moment. Thus, it is possible to improve throughput of the entire system.
In order to carry out communications by sharing one physical channel among a plurality of mobile stations in a time-division manner as described above, the mobile station which carries out communications by using the physical channel concerned must be notified at each TTI (Transmission Time Interval). In the case of the HSDPA system, the notification is made to the plurality of mobile stations by using a shared control channel called HS-SCCH (High Speed-Shared Control Channel).
In the conventional HSDPA system, a base station has set transmission power to be allocated to the HS-SCCH by adding a power offset to transmission power of a dedicated channel (A-DPCH: Associated-Dedicated Physical Channel) accompanying the HS-channels (HS-SCCH and HS-PDSCH).
Additionally, in the conventional HSDPA system, the base station has sometimes set an upper limit value (maximum transmission power) of the transmission power of the HS-SCCH, to prevent excessive allocation of transmission power to the HS-SCCH.
However, in the conventional HSDPA system, because of a difference in interleaving effects due to a large difference between the TTI length of the HS-SCCH and the TTI length of the A-DPCH accompanying the HS-channels (HS-SCCH and HS-PDSCH), there has been a problem in that it is difficult to carry out appropriate transmission power control using the same power offset between cases of small and large moving speeds of the mobile station.
Furthermore, in the conventional HSDPA system, in the case of setting an upper limit value of transmission power of the HS-SCCH so as to prevent excessive allocation of transmission power to the HS-SCCH, there has been a problem in that it is difficult to carry out efficient allocation of the transmission power to HS-SCCH and HS-PDSCH (High Speed-Physical Downlink Shared Channel) for each TTI, because of a deviation in transmission timing of the HS-SCCH from transmission timing of the HS-PDSCH.
Incidentally, the HS-PDSCH is a physical channel used for transmitting the HS-DSCH (High Speed-Downlink Shared Channel).
The aforementioned problems will be described in detail with reference to FIG. 1 and FIG. 2.
FIG. 1 shows an example of a method for allocating transmission power in the case of assuming transmission timing of the HS-SCCH is equal to transmission timing of the HS-PDSCH in the conventional HSDPA system.
FIG. 2 shows an example of a method for allocating transmission power when transmission timing of the HS-SCCH deviates from transmission timing of the HS-PDSCH in the conventional HSDPA system.
Here, transmission power obtained by subtracting transmission power allocated to a non HS-Channel from maximum total transmission power of the base station can be allocated as the transmission power of the HS-SCCH and the transmission power of the HS-PDSCH.
Additionally, in the conventional HSDPA system, the transmission power of the HS-SCCH is determined within a range not exceeding the aforementioned obtained transmission power, and transmission power obtained by subtracting the transmission power of the HS-SCCH from the aforementioned obtained transmission power is allocated as the transmission power of the HS-PDSCH.
In the conventional HSDPA system, the transmission power of the HS-SCCH is controlled by a predetermined algorithm. An upper limit value (maximum transmission power) of the transmission power of the HS-SCCH may be set in order to prevent excessive allocation of transmission power to the HS-SCCH.
In the example of FIG. 1, the transmission timing of the HS-SCCH is assumed to coincide with the transmission timing of the HS-PDSCH. Thus, the transmission power of the HS-PDSCH is set to a large value if the transmission power of the HS-SCCH is small, and the transmission power of the HS-PDSCH is set to a small value if the transmission power of the HS-SCCH is large. As a result, the base station can execute efficient transmission power allocation.
However, according to specifications of the HSDPA system defined in 3GPP, due to the fact that the transmission timing of the HS-SCCH deviates from the transmission timing of the HS-PDSCH by 2 slots as shown in the example of FIG. 2, it is difficult to carry out the aforementioned efficient transmission power allocation.
That is, in the example of FIG. 2, the transmission power of the HS-SCCH and the transmission power of the HS-PDSCH at predetermined TTI are respectively affected by the transmission power of the HS-PDSCH and the transmission power of the HS-SCCH at next TTI. Consequently, efficient allocation of the transmission power becomes difficult.
Specifically, as shown in FIG. 2, the transmission power of HS-SCCH#1 is affected by the transmission power of HS-PDSCH#1, the transmission power of the HS-PDSCH#1 is affected by the transmission power of HS-SCCH#2, and the transmission power of the HS-SCCH#2 is affected by the transmission power of HS-PDSCH#2. Accordingly, in such a case, the transmission power must be determined by considering all future transmission power in order to carry out transmission power allocation similar to that shown in FIG. 1. Realization thereof becomes difficult.
In order to solve the problem, as shown in FIG. 3, a method has been presented to secure the transmission power of the HS-SCCH in a fixed manner by defining an upper limit value (maximum transmission power) of the HS-SCCH, if the transmission timing of the HS-SCCH deviates from the transmission timing of the HS-PDSCH in the conventional HSDPA system.
However, in such a case, compared with the transmission power allocation method as shown in FIG. 1, there is a problem of reduced efficiency in use of transmission power. That is, in the described case, certain transmission power must be secured for the HS-SCCH. Consequently, the transmission power allocated to the HS-PDSCH is reduced, and then throughput of the system and each user will be decreased.